This proposal will test the hypothesis that the direct mTOR effectors and downstream as well as mTOR-independent molecules contribute to cortical neuron hypertrophy and misplacement in Tuberous Sclerosis Complex (TSC). The long term goal of this proposal is to prevent or rescue brain malformations in TSC and other disorders involving the PI3K pathway. TSC is caused by mutations in TSC1 or TSC2 leading to mTOR hyperactivity and developmental malformations associated with seizures and worsening of cognitive and psychiatric deficits. The mTOR inhibitor rapamycin is the only therapeutic option, does not rescue all defects, and has major side-effects emphasizing the need to better understand the molecular basis of TSC and find novel drug targets. We and others have developed mouse models of TSC-associated cortical lesions. Consistent defects found across these models are neuronal misplacement and dendrite hypertrophy. Spine defects remain unclear. Here, we propose to identify some of the molecular players responsible for abnormal placement and morphogenesis (dendrites and spines) of cortical pyramidal neurons. mTOR directly phosphorylates the translational repressor eIF4E binding protein (4E-BP) and p70 ribosomal S6 protein kinase 1 (S6K1). In Aim 1, we will determine whether 4E-BP and/or S6K1 mediate mTOR effects on cortical neuron development. The canonical mTOR activator, Rheb, which is inhibited by the TSC1/TSC2 complex may contribute to some defects independently of mTOR, but this remains speculative. In aim 2, we propose to identify which mTOR- dependent and -independent molecules contribute to cortical defects in TSC. Finally, in aim 3 we will test whether identified molecules (e.g. FLNA) prevent and rescue neuronal defects in TSC. We will use a combination of genetic and pharmacological manipulations as well as translating ribosome affinity purification (TRAP) analysis in collaboration with Dr. Breunig in wild-type and transgenic mice.